This invention relates generally to phase shift keyed (PSK) data packet receivers and particularly to means for stabilizing such a receiver to minimize its pull in and lock up time.
Cable television systems include a head-end that is physically linked to a large number of subscriber terminals by means of individual coaxial cables for providing a wide variety of television-related programming and services for a fee. Each cable subscriber terminal has a uniquely identifiable decoder and the head-end generally includes, or has access to, a computer which, by appropriate data signals, controls the operation of the individual subscriber units in accordance with the type of service selected. The head-end computer may also prepare subscriber bills for the type of service received and perform other "housekeeping" chores.
In at least one such cable system, subscriber terminals automatically transmit information to the cable head-end in direct response to a so-called polling signal from the head-end or indirectly in a "contention" arrangement where subscriber units keep trying to send data to the head-end until a communication takes place. The data is transmitted in a PSK data packet format. Each data packet includes a number of preamble bits to enable lock up of the transmitted signal in the data receiver in the head-end, a group of address bits to associate that data with the particular subscriber unit, a group of bits for conveying the desired information and a small group of bits for error detection. Since the data is transmitted in binary form, phase shift keying of the carrier wave is a very attractive technique with a 0.degree. carrier phase, for example, being used to indicate a logic "1" and an opposite 180.degree. carrier phase being used to indicate a logic "0".
In a PSK system, there is no readily recoverable carrier. While a carrier sample may be sent, that would involve an extra signal and the transmission would be subject to noise problems. Generally, the system includes a phase locked loop (PLL) for locking to the carrier frequency of the data packet. The present invention is directed to a technique for permitting ready lock up of the PLL without requiring long preambles that are detrimental to system throughput.
In the cable system above described, data packets from the individual subscribers are sequentially transmitted and the head-end receiver is subjected to periods of strong signal, where its voltage controlled oscillator (VCO) is locked to the carrier frequency of the PSK data, and periods of no signal where its VCO free runs. The invention involves applying to the PLL input a continuous reference carrier frequency signal of substantially the same frequency as the PSK carrier. The reference carrier keeps the VCO in the PLL close to the lock up frequency and enables rapid lock up when a PSK data packet is received.
The reference signal is applied to a limiter and has an amplitude that is about 15 dB below the level of the lowest amplitude PSK data on the system. When a data packet is received, the reference signal is swamped out by limiter action and the PLL locks to the frequency and phase of the received data packet. When the data packet ends, the PLL locks to the reference frequency. Consequently, the reference frequency signal keeps the VCO close to the frequency of the data packets and is automatically disabled when a data packet is received.